The present disclosure relates to a reference voltage generation circuit, and more particularly to a start-up technique for a reference voltage generation circuit.
Conventionally, bandgap reference circuits are used to reduce variations in circuit characteristics due to changes in power supply voltage and temperature. Such a bandgap reference circuit generates a reference voltage with low dependence on the power supply voltage and temperature.
FIG. 10 illustrates a configuration of a reference voltage generation circuit including a typical bandgap reference circuit 90. In the bandgap reference circuit 90, a control current Ic is controlled so that a voltage (a connection point voltage VD1) at a connection point of a resistance element R1 and a diode element D1 and a voltage (a connection point voltage VD2) at a connection point of resistance elements R2 and R3 are equal to each other. When the connection point voltages VD1 and VD2 are equal to each other, a reference voltage VA is stabilized. The reference voltage VA can be expressed by Equation 1 or Equation 2.
                              V          ⁢                                          ⁢          A                =                              VD            ⁢                                                  ⁢            1                    +                                                                      Δ                  ⁢                                                                          ⁢                  VD                                                  R                  ⁢                                                                          ⁢                  3                                            ·                                                R                  ⁢                                                                          ⁢                  2                                                  R                  ⁢                                                                          ⁢                  1                                            ·              R                        ⁢                                                  ⁢            1                                              [                  Equation          ⁢                                          ⁢          1                ]                                          V          ⁢                                          ⁢          A                =                              VD            ⁢                                                  ⁢            1                    +                                                    R                ⁢                                                                  ⁢                2                                            R                ⁢                                                                  ⁢                3                                      ·                          kT              q                        ·                          ln              ⁡                              (                                                                            R                      ⁢                                                                                          ⁢                      2                                                              R                      ⁢                                                                                          ⁢                      1                                                        ·                                                            Is                      ⁢                                                                                          ⁢                      2                                                              I                      ⁢                                                                                          ⁢                      s                      ⁢                                                                                          ⁢                      1                                                                      )                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
In Equations 1 and 2, “ΔVD” is a difference between a threshold voltage of a diode D1 (i.e., the connection point voltage VD1) and a threshold voltage of a diode D2, “k” is Boltzmann constant, “T” is the absolute temperature, “q” is the amount of electron charge, “Is1” is the saturation current of the diode D1, and “Is2” is the saturation current of the diode D2. In this case, the threshold voltage (VD1) of the diode D1 has a negative temperature characteristic, and the difference voltage ΔVD has a positive temperature characteristic. Because of a virtual short-circuit of a differential amplifier, a potential difference across the resistance element R3 is equal to the difference voltage ΔVD, and a current flowing in the resistance element R1 is “(ΔVD/R3)×(R2/R1).” That is, a voltage having a positive voltage characteristic is generated at the resistance element R1. Since the temperature characteristic of the threshold voltage (VD1) of the diode D1 is canceled out by the voltage generated at the resistance element R1, the reference voltage VA is not dependent on temperature. In Equations 1 and 2, there is no term dependent on the power supply voltage VDD. Thus, it is understood from the equations that the reference voltage VA is also not dependent on the power supply voltage VDD.
Next, the relationship between the reference voltage VA and the connection point voltages VD1 and VD2 will be described with reference to FIG. 11. The connection point voltage VD2 increases with an increase of the reference voltage VA. However, the connection point voltage VD1 becomes constant, after the reference voltage VA exceeds a predetermined voltage level. Thus, there are two points where the connection point voltages VD1 and VD2 are equal to each other. That is, the bandgap reference circuit 90 has an operating stable state where the reference voltage VA is stabilized at a desired voltage level Vx, and a non-operating stable state where the reference voltage VA is stabilized at a voltage level (e.g., a voltage level Vy) lower than the desired voltage level. When the bandgap reference circuit 90 is in the operating stable state, Equations 1 and 2 hold, and therefore, a reference voltage with low dependence on the power supply voltage and temperature can be generated. On the other hand, when the bandgap reference circuit 90 is in the non-operating stable state, the reference voltage VA, the connection point voltages VD1 and VD2, and a control voltage VGN are stabilized at a voltage level close to the ground voltage GND, and a control voltage VGP is stabilized at a voltage level close to the power supply voltage VDD. Thus, the reference voltage VA cannot be maintained at the desired voltage level Vx. Also, the bandgap reference circuit 90 easily becomes in the non-operating stable state at a start of operation (at the time when the power supply voltage VDD is supplied).
Thus, to solve the above-described problems, the reference voltage generation circuit of FIG. 10 includes, in addition to the bandgap reference circuit 90, a current source CS for drawing a current from a connection point of an NMOS transistor and a PMOS transistor to a ground node (see, e.g., page 5 of the specification and FIG. 3 of U.S. Pat. No. 5,686,823). Since a current I91 is drawn by the current source CS, the control voltage VGP reduces, and the control current Ic increases. As a result, the reference voltage VA is forced to increase. Thus, the bandgap reference circuit 90 can be caused to transition from the non-operating stable state to the operating stable state. The reference voltage generation circuit of FIG. 12 includes, in addition to the bandgap reference circuit, a resistance element R for supplying a current from a power supply node to a differential amplifier 900 (see, e.g., “OPERATIONAL AMPLIFIER SPEED AND ACCURACY IMPROVEMENT,” pp. 38-42, 2004, etc.). Since the current I92 is supplied by the resistance element R, the control voltage VGN increases, and the control voltage VGP reduces. As a result, the reference voltage VA can be forced to increase.